1. Field of the Invention
The present invention relates to hard disk drives. More particularly, the present invention relates to a disk drive employing servo zones with banded data zones.
2. Description of the Prior Art and Related Information
A huge market exists for hard disk drives for mass-market host computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive must be relatively inexpensive, and must accordingly embody a design that is adapted for low-cost mass production. In addition, it must provide substantial capacity, rapid access to data, and reliable performance. Numerous mamnfacturers compete in this huge market and collectively conduct substantial research and development at great annual cost, to design and develop innovative hard disk drives to meet increasingly demanding customer requirements.
Each of numerous contemporary mass-market hard disk drive models provides relatively large capacity, often in excess of 1 gigabyte per drive. Nevertheless, there exsts substantial competitive pressure to develop mass-market hard disk drives having even higher capacities. Another requirement to be competitive in this market is that the hard disk drive must conform to a selected standard exterior size and shape often referred to as a "form factor." Generally, capacity is desirably increased without increasing the form factor or the form factor is reduced without decreasing capacity.
Satisfying these competing constraints of low-cost, small size, and high capacity requires a design that provides high format efficiency and high areal storage density. Format efficiency relates to the percentage of available area that is available for stoing user data rather han being consumed by control data, gaps, etc. Areal storage density relates to the amount of data storage capacity per unit of area on the recording surfaces of the disks. The available areal density may be determined from the product of the track density measured radially and the linear bit density measured along the tracks.
The available track density depends on numerous factors including the performance capability of a servo system in the hard disk drive which, among other things, provides for track following, i.e., maintaining alignment of a reading or wiig transducer with respect to the centerline of a desired track. One type of servo system, somees referred to as an "embedded servo" employs servo data on the same disk surface that stores user data to provide signals employed in the operation of the servo system An embedded servo format for the disk surice has the basic characteristic of a plurality of radially-extending servo-data regions (sometimes referred to as "servo wedges") and an interspersed plurality of radially-extending user-data regions. Each user-data region has a plurality of user-data track segments, and each servo-data region has a plurality of servo-data track segments. In accord with another element of an embedded servo format, the servo data include track-identification data used during track-seeking operations, and burst data used during track-following operations. While data are being read in operation of an embedded servo hard disk drive, a transducer produces a time-multiplexed analog read signal that during a revolution of the disk represents servo data during each of a first set of time intervals; and represents user data during each of a second set of time intervals.
The rate at which servo wedges pass under a reading transducer is referred to as the "servo sample rate." The servo sample rate equals the revolution rate ofthe rotating disk multiplied by the number of servo wedges per surface. A high servo sample rate is desirable for the purpose of providing a robust servo system. On the other hand, increasing the servo sample rate generally involves allocating more surface area to servo wedges and thereby adversely impacts surface format efficiency.
The available linear bit density depends on numerous factors including the performance capability of certain circuitry that is commonly referred to as a "read channel." One type of read channel is referred to as a peak-detecting channel; another type is referred to as a sampled-data channel. The type referred to as a sampled-data channel is a category including a partial response, maximum likelihood ("PRML") channel, a EPR4 channel, and a E.sup.2 PR4 channel.
In a hard disk drive havng any of these read channels, the read channel receives an analog read signal from a transducer during a read operation The analog read signal is characterized by a "channel frequency." As used in this arr, "channel frequency" is the reciprocal of a time period "T," where the "T" is the time period consumed while an elemental-length magnet passes under the transducer during a read operation with the disk spinning at a constant angular velocity. In this regard, the length of each magnet recorded along a track as a result of a write operation is, to a first order of approximation, either an elemental length or an integer multiple of the elemental length. Each elemental length magnet can be referred to as a "bit cell" that is defined during a write operation.
The analog read signal always contains some random noise. The analog read signal, and certain other signals produced by processing the analog read signal and that also contain noise, are referred to herein as noise-corrupted signals. One such other noise-corrupted signal is a signal produced by filtering the analog read signal by means of a low-pass filter. Such filtering may reduce but not eliminate noise, and the filtered signal is also noise corrupted. Further signal processing in the read channel provides for producing a digital signal comprising detected symbols, any of which can be in error in representing recovered data Such a digital signal is referred to herein as an error-prone signal.
In a hard disk drive employing a peak detecting channel, digital data are represented in the media by transitions between oppositely magnetieed bit cells. Provided that the transitions between oppositely magnetized bit cells do not unduly interfere with each other, each such transition causes a peak in the analog read signal, and a peak-detecting channel employs a peak detector that detects such peaks, and produces digital signal in the form of a serial binary-valued signal that is an error-prone signal for numerous reasons. One reason why the peak detector produces an error-prone signal is random noise; this source of error presents a problem for any type of channel. Another reason relates to interference between adjacent transitions. Interference between such transitions is referred to as intersymbol interference and adversely affects performance of a peak detetecting channel increasingly as a fimction of channel rate.
A sampled-data channel employs sampling circuitry that samples a noise-corrupted analog read signal to produce a sequence of noise-corrupted samples. The samples so produced are provided in sequence to a detector such as a so-called "Viterbi detector" that internally produces error-prone symbols and maps the internally-produced error-prone symbols to binary-valued error-prone symbols. In a PRML channel, such internally-produced error-prone symbols are often referred to as: "-1; "0"; and "+1"; and the binary-valued error-prone symbols are supplied to a deserializer to produce a parallel-by-bit digital signal.
A contemporary hard disk drive utilizes zone banding for data to provide high capacity. An advantage of zone banding resides in providing higher linear bit density recording. In older disk drives that did not employ zone banding, the data was recorded at substantially the same channel rate for every track on the recording surface. Because the circumference of each track is a function of radius, and because the same channel rate is used in such older drives, the linear bit density changes as a function of track radius. In a contemporary embedded servo disk drive employing zone banding for data, the channel frequency for data changes from one band to another, with the highest channel frequency being used for the outermost zone band.
The emergence of the sampled-data channel in disk drive applications has enabled higher linear bit densities. However, a servo-data track segment demands a lower raw bit error rate (BER) than a user-data track segment in order to efficiently process the servo data used during track-seeking and track-following operations. The raw BER refers to the error rate for data detected by the read channel which is used by the servo system without the benefit of using an ECC correction system to correct errors in a servo data sequence. Although the lower raw BER requirement is a limitation on increasing the linear bit density in the servo-data track segment, reading the servo data without ECC correction reduces the processing time for performing the track-seeking and track-following operations. Also, providing ECC data in the servo-data track segment to allow the servo system to operate with an increased raw BER has a disadvantage of reducing disk area available for data storage. Accordingly, embedded servo disk drives commonly employ constant frequency servo sectors and banded data zones. Such a disk surface format is inefficient due to the constant frequency servo sectors having lower linear bit density at the OD than at the ID, thereby potentially reducing disk area available for data storage.
U.S. Pat. No. 5,384,671 to Fisher (the '671 patent) discloses a single frequency for both user data and servo information within a disk data zone of a disk drive. However, the frequency for the user data is limited by performance specifications such as low error rates for the servo data in the disk data zone. Such a limitation on the frequency for the user data can limit the areal storage density in the disk drive.
There is a need for a disk surface having a high format efficiency and high areal storage density.